The present invention relates to a method for crystallizing sugars and polyols, in particular with reference to sorbitol. The invention also relates to the crystallized sorbitol obtained with such a method.
The following description mainly concerns a method for the crystallization of sorbitol, since, among all the sugars and polyols compounds, sorbitol is the one which implies the greatest technical difficulties as far as crystallization is concerned.
Sorbitol has been broadly used as a plasticizer and a filler in many products of the pharmaceutical and confectionery industry, where nowadays is used as a sweetener or as an excipient which xe2x80x9caidsxe2x80x9d the tableting of the powder formulations containing it.
Because of its high hygroscopicity, sorbitol must be crystallized at a crystallization degree of at least 80% and must be preferably in the (gamma) crystallization form in order to be used in the above mentioned fields.
As it is known, sorbitol is usually obtained through catalytic hydrogenation of glucose, at the end of which, it is in the form of a highly viscous melted mass (or magma) with a concentration of about 70-72% of the dry material.
In order to be crystallized in the desired form, the thus obtained sorbitol must be concentrated up to values of 99÷99.7% of dry material, values at which sorbitol maintains the starting form of melted (or magma).
In order to prepare sorbitol with a high degree of crystallinity, the procedure usually implies cooling the melted and hot sorbitol mass, after adding a suitable quantity of sorbitol crystals or seeds.
During the cooling, the melted mass of sorbitol is usually kept under constant mixing.
The crystallized sorbitol is then subjected to fine crushing and to screening.
Despite the high degree of crystallization, the obtained sorbitol shows the disadvantage of a poor flowability and the tendency to pack when it is subjected to pressure, thus causing problems when the formulations which contain such sorbitol must be tableted.
According to improved techniques of the above mentioned method, the crystallized sorbitol melted mass is cold extruded and in such a way a higher degree of crystallization and a product more suitable for tableting are obtained.
But this technique shows an acknowledged control difficulty for a desired uniformity in the treatment of the sorbitol mass and, therefore, gives rise to a non-constant quality in the final product.
Furthermore, if the extrusion conditions are exasperated or if they are not controlled within a close range of experimentally predetermined values, a vitrification of the product can occur at a certain degree, with a subsequent qualitative decay of such a product and with a reduction of the already low flowability.
Another broadly used technique is substantially based on a batch, prolonged treatment, characterized by a slow mixing of the sorbitol melted mass plus crystallized sorbitol added as a seed, in big mixers appropriately equipped (marl treatment).
Besides the problems related to a batch production, this technique shows the acknowledged inconvenience of the cross contamination. The seriousness and the frequency with which such a contamination occurs make compulsory a thorough cleaning of the mixers at the end of each operative phase, in order to prevent fermentation processes, which would be otherwise inevitable.
The problem underlying the present invention is that of providing a method for a continuous crystallization of sorbitol, which enables to obtain sorbitol with a high degree of crystallization (or with great amounts of crystallized gamma-sorbitol), overcoming all the disadvantages mentioned with reference to the prior art, and also enabling the crystallization of other polyols and sugars.
According to the invention, this problem is solved by the method in accordance with claim 1.
For the steps of formation of the thin layer and of granulation of the material which forms such a layer, an apparatus manufactured by VOMM IMPIANTI E PROCESSI S.R.L. of ROZZANO (Milan) is advantageously used. This apparatus is available on the market with the name TURBOCRYSTALLIZER and includes a cylindrical tubular body, which is closed at its opposite ends by respective end walls and has an inner wall kept at a prefixed temperature by means of a jacket formed in the cylindrical body, a powered rotating shaft, which extends axially in said cylindrical body and is rotatably supported by its opposite end walls, said shaft being provided with radial blades, which are helically arranged and extend nearly up to said inner wall, at least an inlet opening for a flow of material to be treated and at least an outlet opening for the obtained product.
For the sake of clarity and conciseness, such an apparatus will be named turbocrystallizer in the following description and claims.
When the above apparatus is used, the method of the present invention comprises the steps described in claims 2-8.
With particular reference to the production of crystalline sorbitol, the method according to the present invention comprises the steps of
feeding a first flow comprising melted sorbitol with at least 99% of dry material and a second flow of crystallization seed consisting of crystallized sorbitol, in a turbocrystallizer having the inner wall thermostated at a temperature comprised between xe2x88x9215xc2x0 C. and 5xc2x0 C., and having a bladed shaft rotating at a speed of 400-1200 r.p.m.;
intimately mixing said flows, by simultaneously centrifuging them against said thermostated wall, with the formation of a turbulent, tubular, thin layer;
advancing said thin layer along said thermostated wall with simultaneous and continuous granulation of the mixture of said flows, forming said layer;
discharging a continuous flow of crystallized granular sorbitol from said turbocrystallizer, and
cooling to room temperature said granular sorbitol after a prefixed maturation time.
Advantageously, the above-mentioned flows are fed in the turbocrystallizer in a independent way one from the other.
The weight ratio between said crystallization seed (crystallized sorbitol) and said melted sorbitol entering the turbocrystallizer, is comprised in the range between 3:1 and 0.5:1 and preferably between 1.5:1 and 1:1.
The obtained granular sorbitol was shown by analysis to contain more than 95% crystallized gamma-sorbitol.
This result is surprising if it is considered that the residence time in the turbocrystallizer varies between 20 and 120 seconds and it is mainly due to the basic idea of operating (mixing, crystallizing, granulating) in a thin layer.
A further and even more surprising result consists in that each sorbitol granule shows an outer surface having a xe2x80x9cvitrifiedxe2x80x9d physical aspect, even though it is completely crystallized in the gamma form, as repeated tests have demonstrated.
This unexpected physical characteristic of the crystallized gamma-sorbitol of the present invention implies a double advantage in comparison with the prior art: a remarkable flowability and a substantial reduction, or even a complete cancellation, of the tendency to pack also when it is subjected to remarkable pressures. All this provides an increased suitability for tableting operations.
Advantageously, the melted sorbitol is fed in the turbocrystallizer at a temperature comprised in the range 85xc2x0 C.-120xc2x0 C., while the crystallization seed (crystallized sorbitol) is fed at room temperature.
The melted sorbitol is preferably obtained directly from the concentration phase of sorbitol produced by catalytic hydrogenation of glucose, while the crystallization seed flow is made by a part of the crystallized gamma-sorbitol, recycled after the maturation step.
The features and the advantages of the invention will be further clarified by the following description of some exemplary embodiments of the present crystallization method.